1. (a) Field of the Invention
The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same, and particularly to a non-aqueous electrolyte for a lithium secondary battery having good safety and good storage characteristics at a high temperature, and a lithium secondary battery comprising the same.
2. (b) Description of the Related Art
As lithium secondary batteries comprising non-aqueous electrolytes are known to generate a high voltage, and have a high energy density and good storage characteristics as well as good operability at a low temperature, they are widely applied to portable personal electronic devices. Further, active research has been undertaken to provide batteries having a capacity high enough to apply as an energy storage device for electric vehicles or midnight time electricity. Recently, batteries have come to require thinness as well as a high capacity, so the demand for polymer batteries or laminated thin lithium secondary batteries has increased.
Since most conventional solvents have a low flash point and high inflammability, they may cause dangers such as fire, explosions, and so on. In order to ensure safety, many schemes have recently been suggested. For example, Japanese Patent Laid-open Publication No. H10-189043 discloses a non-aqueous electrolyte that is prepared by adding a halogenated carbonate, thereby obtaining an electrolyte that is able to reduce the risk of combustion and that has good performance at both a high temperature and a low temperature, and good cycle-life characteristics.
Japanese Patent Laid-Open Publication No. H11-40199 also discloses a non-aqueous electrolyte mixed with a halogenated carbonate, and the obtained battery is able to operate a safety valve upon undergoing an increase in internal pressure thereof, so that its safety is ensured.
However, a lithium secondary battery in which a non-aqueous electrolyte is mixed with a halogenated carbonate causes problems in that a film formed on the surface of the negative electrode decomposes to generate a gas, which remarkably increases the internal pressure thereof when the battery is stored around 60° C. for several days. Particularly, in a case of a polymer battery or a laminated thin lithium secondary battery, it is a serious problem that the battery thickness increases due to the decomposition gas.
In addition, in the case of a polymer battery or a laminated thin lithium secondary battery, internal shorts are repeatedly caused by excessive expansion of the battery volume due to an overcharge. Particularly, when the battery is overcharged by a large current from a discharged state, it is hard to ensure safety since the internal shorts easily occur due to the lithium deposition.